In wafer-to-wafer bonding technology, various methods have been developed to bond two package components (such as wafers) together. Some wafer bonding methods include fusion bonding, eutectic bonding, direct metal bonding, hybrid bonding, and the like. In fusion bonding, an oxide surface of a wafer is bonded to an oxide surface or a silicon surface of another wafer. In eutectic bonding, two eutectic materials are placed together, and a high pressure and a high temperature are applied. The eutectic materials are hence melted. When the melted eutectic materials solidify, the wafers bond together. In direct metal-to-metal bonding, two metal pads are pressed against each other at an elevated temperature, and the inter-diffusion of the metal pads causes the bonding of the metal pads. In hybrid bonding, the metal pads of two wafers are bonded to each other through direct metal-to-metal bonding, and an oxide surface of one of the two wafers is bonded to an oxide surface or a silicon surface of the other wafer.